Auxin is an essential hormone for many aspects of plant growth and development. The general goal of this proposal is to elucidate the molecular mechanisms by which auxin regulates various developmental processes. We approached key questions in auxin biology by first elucidating the molecular mechanisms of auxin biosynthesis. We have unambiguously established the first complete tryptophan (Trp)-dependent auxin biosynthesis pathway. Plants use a two-step pathway to convert Trp into Indole-3-acetic acid (IAA), the main auxin in plants. Trp is first converted to indole-3-pyruvate (IPA) by the TAA family of amino transferases and subsequently, IPA is converted to IAA by the YUC family of flavin-containing monooxygenases. The TAA/YUC pathway is the major auxin biosynthesis pathway in plants. By screening for genetic modifiers of auxin biosynthesis mutants, we firmly established a novel signal transduction pathway for auxin-mediated organogenesis. The known components in the signaling pathway are: the protein kinase PID, the scaffolding protein NPY1, and the transcription factor ARF5. Our progress in auxin biosynthesis enables us to modulate auxin levels in plants with spatial and temporal control, thus allowing us to address key questions in auxin biology from a different perspective. The main aims of the proposed studies are: (1) Assess the relative contributions of local auxin biosynthesis and polar auxin transport to Arabidopsis root development; (2) Elucidate the roles of the protein kinase SKA1 in auxin-mediated organogenesis; (3) Genetically delineate the signaling pathway by which auxin controls flower development. The proposed experiments will lead to a more complete picture of the roles of localized auxin biosynthesis in plant development. The proposed work will also provide significant new insights into the signaling mechanisms that control complex developmental processes.